My invention relates to a recording method in magnetic tape data storage and retrieval systems employing a rotary head assembly, such as digital audio tape recorders (DATs) and video tape recorders (VTRs). Typically, the recording method of my invention lends itself to use with those DATs which are adapted for the storage and retrieval of non-audio data, with a read-after-write monitoring capability, for use as external storage devices of computers.
Japanese Unexamined Patent Publication No. 62-177701, laid open to public inspection on Aug. 4, 1987, represents an example of known rotary head magnetic data storage and retrieval system providing for substantially concurrent recording and monitoring of digital data on a tape cassette. It teaches the use of a rotary head assembly including a rotary drum to which there are mounted both a pair of read/write heads and a pair of monitor heads. The pair of read/write heads are disposed in diametrically opposite positions on the rotary drum. The pair of monitor heads are also disposed in diametrically opposite positions on the drum. An angular difference of 90 degrees exists between the positions of the two pairs of heads on the drum.
In this prior art system, as the read/write heads create a series of slanting record tracks on the magnetic tape, the monitor heads substantially concurrently scan the tracks for monitoring the recorded data. In event the output signals of the monitor heads indicate that data is not being recorded properly, the host computer commands the rewriting of the data.
Generally, in rotary head DAT (R-DAT) systems having a substantially concurrent monitoring capability, the pair of read/write heads alternately form tracks so as to overlap the preceding tracks. Such overlapping results in the ultimate creation of record tracks of reduced width. Hereinafter in this specification, and in the claims appended hereto, the term "overwriting" will be used to mean the writing mode wherein the tracks formed by one of the pair of read/write heads lap over the marginal edge portions of the tracks formed by the other. The word "rewriting", on the other hand, will be used to mean the writing of new data on preformed data tracks.
The monitor heads of most conventional R-DAT systems scanned the tracks before they were overwritten. The record tracks eventually formed by overwriting were narrower than those scanned by the monitor heads. Such conventional R-DAT systems enable the nearly simultaneous monitoring of the recordings being made. However, they have often failed to ascertain whether the data is stored on the overwritten tracks with a sufficient quality margin or not. Some data errors on the overwritten tracks present little or no serious problem in DAT decks used for the recording and playback of digitized audio signals. The audio DAT decks are usually equipped to automatically correct such errors during the process of playback to such an extent that the reproduced sound will be practically free from audible distortions. However, recording errors can be fatal in the use of the DAT decks as external storage devices of computer systems.
A known solution to this problem has been a rotary head assembly wherein a pair of read/write heads and a pair of monitor heads are so arranged on a rotary drum that the monitor heads scan the data tracks created by the read/write heads after they have been overwritten. The monitor heads are spaced from the read/write heads not only circumferentially but also axially of the rotary drum. The monitor heads of this known rotary head assembly can be positioned in centerline alignment with the overwritten tracks and so can be utilized for the accurate tracking control of the read/write heads in writing data over the prerecorded tracks.
The prior art rotary head assembly has proved to present certain difficulties unless the read/write heads and the monitor heads are disposed exactly in predetermined positions on the rotary drum. It is difficult, or practically impossible, to meet this requirement in the quanity production of such rotary head assemblies. Some variations in the relative positioning of the read/write heads and monitor heads of such rotary head assemblies are unavoidable within the ranges of the predefined tolerances.
Let us consider a magnetic tape cassette on which data has been written in one DAT deck employing the noted prior art rotary head assembly and which is to be rewritten in another. The read/write heads of the second mentioned DAT deck have often been not positioned in precise centerline alignment with the existing tracks of the tape cassette because of the difference between the relative placements of the read/write heads and monitor heads of the rotary head assemblies in the two DAT decks.
More specifically, if the read/write heads and the monitor heads of the rotary head assembly in the second DAT deck are spaced too much from each other in the axial direction of the rotary drum (i.e. in the transverse direction of the data tracks on the tape), one of the read/write heads have not been positioned to cover the complete width of the first of the target group of existing data tracks to be rewritten. Both old and new data have then come to coexist on this first track, and both data have been read by the associated monitor head. If the read/write heads and the monitor heads are spaced too little, on the other hand, then one of the read/write heads have not been positioned to cover the complete width of the last of the target group of existing tracks. The accuracy of the data read from this last track has been unreliable.